Powder metallurgy is a well established process for the manufacture of a wide range of products for various applications. In its simplest form, the process involves pouring fine powders into a precision metal mold which has moveable elements (FIG. 1) and then applying pressure to the powder to form a "compact". The compact is then ejected from the mold by a relative upwards motion of the bottom tool element (punch). Holes can be formed in the compact by use of "core-pins". The compact is then subjected to a thermal process called "sintering" which involves heating the compact in a temperature controlled furnace under a protective atmosphere to effect powder particle bonding and alloying which results in a strong metal product that can be used for structural and mechanical purposes.
It is also well known that the physical and mechanical properties of the "sintered product" are highly dependent upon its density. Since both static and dynamic strength are highly valued properties of materials, there has been extensive work in both academic and industrial arenas to increase the density at low cost. There are several costly ways of achieving this high density goal: double processing which involves restriking the sintered product and then resintering it, hot forging the sintered product, and recently "warm pressing" of powder mixes involving special expensive lubricants and binder powder additives plus a system for precision heating of the powder mixture prior to compaction in warm tooling.
The present invention, however, is an improvement on another approach which involves lubrication of the vertical surfaces of the mold elements (tools). This invention allows elimination of powder lubricants normally added to the mix to facilitate ejection of the compact from the mold to occur without scoring or galling of the tools from cold welding of metal powder particles to metal tool elements. Elimination of the pressing lubricants which are light soap-like powders such as an organic stearate, clears the way for extra metal powder densification at high compacting pressures.
Mold wall lubrication is not new. In fact, it has been practiced commercially at Zenith Sintered Products, Germantown, Wis. U.S.A. since before 1985 under the trade name Z95 Plus. This, however, involved a liquid lubricant spray onto the tool surfaces. A drawback to the process is that the resulting compact surface is wet, and this collects and holds loose powder which bonds to the compact in the sintering stage. The result can be unacceptable quality products. The washing of compacts has been used to overcome this problem, but the washing process has its own problems. The liquid carrier medium also presents problems since it must be volatile yet meet stringent safety regulations.
The search for a dry powder sprayed on mold coating was therefore a direction of research. Recently a process involving charging the lubricant powder particles electrostatically and spraying them onto the mold which is electrically grounded has been developed and the results published widely. A major limitation with this process is with respect to the depth of mold that can be effectively coated to permit ejection of a compact under high pressing pressures. A variety of lubricant powders were sprayed onto mold wall surfaces using a "Tribostatic Sprayer" which was attached to a production compacting press using production tooling to make a right cylinder of approximate dimensions 1.5 inches outer diameter, 1.0 inches inner diameter. It was found by experimentation that at high pressing pressures (above 50 tons per square inch) the maximum density achievable of an iron-carbon-copper powder mix was limited to 7.25 grams per cubic centimeter and the vertical length (height) was limited to about 0.5 inches. The limiting mechanism governing the height of the compact was the removal of powder lubricant from the top half of the mold surface during the powder compaction stage. Since the powder height is about halved during compaction, the top half of the mold wall, past which the compact must be ejected, is dry and unlubricated prior to ejection. This leads to scoring and galling of the mold surface on ejection. It was confirmed that wet spraying of the mold surface did not suffer from this effect, since a wet residue is left on the upper half of the mold wall during compaction, that provides lubrication during compact ejection.